1. Field of the Invention
This invention relates to a continuous process for the production of aqueous polyurethane dispersions by continuous conversion of solutions of hydrophilically modified polyurethanes or hydrophilically modified NCO-prepolymers in an optionally aqueous organic solvent into an aqueous dispersion with continuous removal of the solvent present in the dispersion by distillation and to the use of the aqueous dispersions thus obtained as a coating composition or as an adhesive.
2. Description of the Prior Art
Processes for the production of aqueous polyurethane dispersions, which are understood to include both aqueous dispersions or suspensions of pure polyurethanes and polyurethane ureas, are known and described, for example, in the following literature references: DE-PS 880,485, DE-AS 1,044,404, US-PS 3,036,998, DE-PS 1,178,586, DE-PS 1,184,946, DE-AS 1,237,306, DE-AS 1,495,745, DE-OS 1,595,602, DE-OS 1,770,068, DE-OS 2,019,324, DE-OS 2,035,732, DE-OS 2,446,440, DE-OS 2,345,256, DE-OS 2,427,274, US-PS 3,479,310 and Angewandte Chemie 82, 53 (1970) and Angew. Makromol. Chem. 26, pp. 85 et seq. (1972).
Of the processes mentioned above, the "acetone process," analogous to the teaching of DE-OS 1,495,745 (US-PS 3,479,310) and DE-OS 1,495,847 (1,076,688), is particularly important. In this process, an NCO prepolymer is initially prepared, then dissolved in an inert solvent and finally chain-extended in solution to form the relatively high molecular weight polyurethane. The incorporation of the hydrophilic groups required for dispersion is preferably achieved either by incorporating diols containing ionic, potentially ionic or non-ionic hydrophilic groups in the prepolymer or by using corresponding amines as chain-extending agents. Dispersion is carried out discontinuously in vessels equipped with stirrers and, optionally, baffles. The solvent used is generally distilled off from the vessel immediately after dispersion in water.
Despite the excellent properties of the products obtained, the "acetone process" is attended by serious disadvantages. Because of the need to use a solvent, which in general is added to the prepolymer before chain extension and dispersion in the reaction vessel and is distilled off from the reaction vessel after dispersion, the space available inside the vessel can never be fully utilized. This has an adverse effect upon the volume-time yield and increases production costs. In addition, the often prolonged exposure of the dispersion to elevated temperature during removal of the solvent by distillation can give rise to problems in the case of temperature-sensitive dispersions. Moreover, the long distillation times often have an adverse effect upon the condition of the dispersed particles, particularly their degree of swelling. Further, in order to be reused, the solvent distilled off generally has to be freed from residues of water in a subsequent energy-intensive distillation step because the water remaining can react in undesirable secondary reactions with the NCO-groups of the prepolymer during its dissolution.
Processes for the production of aqueous polyurethane dispersions by continuous dispersion are also known. DE-OS 2,260,870 for example describes the use of special mixing reactors designed on the basis of developing cellular flow. Reactors such as these represent technically elaborate constructions in which both the chain-extension step and also the dispersion step are carried out, the position of the dispersion zone determining the chain-extension time. The size of the dispersion zone determines the particle size and particle size distribution of the dispersion. However, the size of the dispersion zone depends to a very large extent upon the stirrer speed and upon the throughput. On the other hand, the stirrer speed affects the development of the cellular flow. Accordingly, the system depends on the strict adherence to several parameters, which in practice is difficult to do over the long term. In addition, the solvent used in DE-OS 2,260,870 is separated off by means of a thin-layer evaporator. Although a thin-layer evaporator provides for very short residence times and favorable heat transfer values, it is attended by the disadvantage that film-forming dispersions show a tendency towards caking.
According to DE-OS 2,311,635, aqueous polyurethane dispersions are continuously prepared using impeller homogenizers in which chain-extension and dispersion take place at the same time. For dispersion, the prepolymers have to be introduced as a melt which generally requires relatively high temperatures. Further disadvantages include chain-extension in non-homogeneous phase and the use of emulsifiers so that coarse suspensions are generally obtained. According to DE-OS 2,344,135, aqueous polyurethane dispersions are continuously prepared using nozzles functioning on the countercurrent injection principle. In this case the chain-extending agent may be added before, during or after mixing of the prepolymer with the aqueous phase. In every case, however, the material discharged from the mixing chamber has to be stirred to achieve chain-extension. In other words the actual chain-extending reaction again takes place in non-homogeneous phase so that coarse suspensions are generally obtained.
Accordingly, an object of the present invention is to provide a new process for the continuous production of aqueous polyurethane dispersions which is not attended by any of the disadvantages referred to above. This problem was solved by the process according to the invention which is described in detail hereinafter.